1. Field of the Invention
The present invention relates to a radiation image recording apparatus, which records an image as an electrostatic latent image by use of an image recording medium including an electric accumulator that accumulates electric charges in a quantity corresponding to an electromagnetic wave irradiated for recording electric charges as a latent image. The present invention also relates to the image recording medium.
2. Description of the Related Art
A conventionally-known method for use in radiation photography is a method utilizing, for example, a solid state radiation detector (an electrostatic recorder) having a photoconductor such as a selenium plate, which is sensitive to radiations such as X-rays, as an image recording medium including an electric accumulator that accumulates electric charges in a quantity corresponding to an electromagnetic wave irradiated for recording as electric charges for a latent image, in order to reduce radiation dosage of a test subject and to enhance diagnostic performances in medical radiography. In the foregoing method, radiation is irradiated onto the detector to allow the electric charges in a quantity corresponding to the irradiated radiation to be accumulated in the electric accumulator inside the detector, whereby a radiation image is recorded as an electrostatic latent image. Moreover, the radiation image is retrieved out of the detector by scanning the detector, which records the radiation image, with a laser beam or a line light source (such as U.S. Pat. No. 4,378,318, for example).
A solid state radiation detector has been disclosed in Japanese Unexamined Patent Publication No. 2000-284056, which is capable of combining high-speed response upon reading out and efficient extraction of signal charges. The solid state radiation detector disclosed in Japanese Unexamined Patent Publication No. 2000-284056 includes a first electrode layer which is transmissive of radiation for recording or light emitted due to excitation by the radiation, a recording photoconductive layer which takes on conductivity by receiving irradiation of the radiation for recording or the light, an electric accumulator which accumulates electric charges in a quantity corresponding to the irradiated radiation dosage as electric charges for a latent image, a retrieving photoconductive layer which takes on conductivity by receiving irradiation of an electromagnetic wave for retrieval, and a second electrode layer to which the electromagnetic wave for retrieval is irradiated, in which the foregoing constituents are stacked in accordance with the order of enumeration. Here, the second electrode layer further includes a first stripe electrode composed of multiple line electrodes that transmit the electromagnetic wave for retrieval, and a second stripe electrode for shielding the electromagnetic wave for retrieval, in which the first stripe electrode and the second stripe electrode are alternately arranged substantially in parallel.
In the above-described solid state radiation detector, if the radiation which passed through an imaging object is irradiated onto the first electrode layer of the above-described solid state radiation detector in the state that a voltage has been applied such that the first electrode layer is set to negative electric potential and the second electrode layer is set to positive electric potential, then charge pairs are generated on the recording photoconductive layer corresponding to the dosage of the radiation owing to irradiation of the radiation that passed through the first electrode layer. Accordingly, negative electric charges are accumulated in the electric accumulator as the electric charges for a latent image, whereby the radiation image is recorded as an electrostatic latent image. Moreover, in this event, if recording is performed in the state that the first stripe electrode and the second stripe electrode are set to the same electric potential by connecting the two stripe electrodes together, then the electric charges for a latent image are accumulated in the electric accumulator in positions corresponding to both electrodes.
Then, when the electromagnetic wave for retrieval is irradiated onto the second electrode layer of the above-described solid state radiation detector, then the electromagnetic wave passes through the first stripe electrode and is irradiated onto the retrieving photoconductive layer, whereby charge pairs are generated on the retrieving photoconductive layer. Among these charge pairs, positive electric charges thereof are coupled with the negative electric charges which were accumulated in the electric accumulator and negative electric charges thereof are coupled with positive electric charges which are charged on the first stripe electrode and the second stripe electrode. Accordingly, the electrostatic latent image is retrieved.
In addition, a method of enhancing retrieval efficiency in the course of recording with the above-described solid state radiation detector has been proposed. The method includes the steps of applying a direct-current voltage between the first electrode layer and the second stripe electrode layer, controlling distribution of an electric field formed in accordance with application of the voltage by applying a given controlling voltage to the first stripe electrode, and localizing the electric charges for a latent image accumulated in the electric accumulator into the electric accumulator in positions corresponding to the second stripe electrode to which current detection amplifiers are connected. Another method has been proposed, in which the electric charges for a latent image are localized and accumulated in the electric accumulator in the position corresponding to the second stripe electrode, not by applying the given controlling voltage to the first stripe electrode as described above, but by setting the electric potential of the first stripe electrode to an electric potential closer to the first electrode layer than the second stripe electrode by means of setting the first stripe electrode to an open state. Furthermore, in the above-described method, a similar effect is obtained when the voltages to be applied to the first stripe electrode and to the second stripe electrode are inverted from the foregoing. In other words, it is possible to localize and accumulate the electric charges for a latent image in the electric accumulator in the position corresponding to the first stripe electrode, by applying a direct-current voltage between the first electrode layer and the first stripe electrode, and then controlling distribution of an electric field formed in accordance with application of the voltage either by applying a given controlling voltage to the second stripe electrode or by setting the second stripe electrode to an open state.
However, when the radiation image is recorded by use of the above-described solid state radiation detector, the direct-current voltage for formation of the electric field is applied between the first electrode layer and at least one of the first stripe electrode and the second stripe electrode. In this event, the first stripe electrode and the second stripe electrode cannot ignore resistance possessed by the electrodes owing to the shapes thereof. Accordingly, a relatively large electric current flows on the first stripe electrode and the second stripe electrode owing to application of the high direct-current voltage, and the electric current causes potential differences on both ends of the respective stripe electrodes. Accordingly, if an end of each line electrode of the first stripe electrode is set as a grounded end and image signal obtaining means such as a current detection amplifier is connected to an end of the second stripe electrode in a position opposite from the grounded end (the second stripe electrode is grounded via an imaginary short of the current detection amplifier), for example, then the grounded end of the first stripe electrode and an open end of the second stripe electrode become adjacent and an open end of the first stripe electrode and the connecting end of the second stripe electrode to the image signal obtaining means (a grounded end) become adjacent. Accordingly, there is a risk of inducing discharge breakdown of the electrodes due to potential differences between the first stripe electrode and the second stripe electrode at the foregoing adjacent portions. Moreover, the risk of the discharge breakdown is increased in the case of recording while applying the controlling voltage to the first stripe electrode or the second stripe electrode or in the case of recording while setting the first stripe electrode or the second stripe electrode to the open state, because the above-described potential difference may be further increased in those cases.
Meanwhile, it is conceivable to provide an insulating layer for insulating the first stripe electrode and the second stripe electrode for the avoidance of the discharge breakdown. However, if the insulating layer is provided, the electric charges for a latent image will be retrieved through the insulating layer. Accordingly, there is a problem of inducing degradation of signal detection efficiency.